1. Field of the Invention
This invention relates to a diagonal propagation, digital multiplier of a type intended for multiplying a first factor (x(n)) by a second factor (M), with either factor being expressed by a binary number.
The field of application of the invention is related, in particular but not solely, to digital FIR (Finite Impulse Response) filters and reference will be made, throughout this description, to that field of application for convenience of illustration.
2. Discussion of the Related Art
As is known, digital filters are devices designed to convert a sampled signal x(n) being input thereto into another sampled signal y(n) having predetermined frequency response characteristics. A sampled signal is, of course, a digital signal encoded as a binary number of N digits or bits on which the filter accuracy or resolution is dependent.
The prior art teaches filters in integrated circuit form that comprise digital multipliers and adders arranged so as to produce, for each input, an output y(n) given by the sum of the present and past input samples each multiplied by a respective coefficient d(i), to be obtained from the filter pulse response.
For example, a typical filter of conventional construction uses a chain of N-1 adders, each provided with first and second inputs, and an output. The output of each one adder is connected to the first input of the next, through a delay block (T), and the second input of each adder is connected to the output of a corresponding digital multiplier (X). The various coefficients d(i) of response to the filter pulse are associated directly with the multipliers (X).
According to the prior art, the multipliers have been embodied by circuit structures which are quite complex and slow during processing.
Structures of that kind are described, for instance, in Joseph Cavanagh, "Digital Computer Arithmetics", McGraw-Hill, pages 137-235.
The slow speed of prior art multipliers imposes a serious limitation on the implementation of fast digital filters; in fact, the most widely used commercial designs can only perform satisfactorily when operated at a low frequency.
Another limitation of current designs results from the extreme difficulty of implementing so-called adaptive filters, i.e. filters for which the value of one of the multiplicative factors, such as that of the filter coefficients d(i), for example, can be readily changed.